The present invention relates generally to telecommunication transceivers, and more particularly, to a semiconductor avalanche photodiode for use in a transceiver and a method of fabricating the same.
Telecommunication transceivers are utilized in various applications to transmit and receive communication signals in telecommunication networks. Fiberoptics are used as a transmission medium between the telecommunication transceivers for various transmission reasons including low noise interference, high-speed data transmission rates, and large multiplexing capabilities. In order for the telecommunication transceivers to receive the communication signals transmitted via light over fiberoptic cable, photodetectors are utilized.
Photodetectors transform light energy into electrical energy. Reverse saturation current is controlled by light intensity that shines on the photodetectors. The light generates electron-hole pairs, which induce current. The resulting current is directly proportional to the light intensity.
The use of fiberoptics introduces practical, feasible, and functional requirements. The photodetectors are preferably semiconductor diodes that are inexpensive due to large quantity requirements, reliable, and capable of relatively high yields. It is also desirable for the photodetectors to provide low noise or low dark current and be amendable to high volume production. One type of photodetector, that is commonly used, that has some of these characteristics including desired detection sensitivity, is an avalanche photodiode (APD).
APDs allow a light induced carrier to be multiplied through an application of a reverse bias p-n junction. An APD is biased near a breakdown region, which causes a cascading effect. As charge is accelerated by a high bias potential that is applied across the p-n junction of the APD, absorption of an incident photon is amplified and charge is generated in amplified proportion to the light intensity.
Current approaches to fabricating reliable planar indium galium arsenic (InGaAs) APDs or planar indium phosphide (InP) APDs utilize either epitaxial growth or double diffusion methods that each have significant limitations.
Epitaxial regrowth methods require that a partially fabricated APD or wafer be removed from an epitaxial growth chamber, exposed to processing including photomasking and etching, and then be placed into a growth chamber for subsequent overgrowth of InP. When using an epitaxial regrowth method, a difficulty arises in returning a wafer impact surface to a pristine or low defect condition. Processing the wafer outside of the epitaxial growth chamber exposes it to particulates, processing chemicals, and other impurities known in the art. Also resulting APDs, of an epitaxial regrowth method, have poor dark current and noise performance characteristics.
Double diffusion methods require careful attention to diffusion parameters and require that a p-n junction be terminated with one or more floating field rings. The floating field rings extend a top surface area of an APD and increase capacitance of the p-n junction without increasing an optically sensitive area or active area. The floating field rings are located at a fixed distance outside the active area so the total junction area and capacitance is increased. Increased p-n junction capacitance undesirably limits bandwidth of the APD. Also, in using a double diffusion method, spatial uniformity of individual layers need to be maintained in order to form a photodiode with relatively uniform process yields across the APD.
Another disadvantage with existing APDs is that there is a large amount of time and costs involved in fiber-optic alignment to the APDs. Since the APDs are high frequency devices that have small capacitances, they have relatively smaller active areas. Small active area devices require active alignment, monitoring the detector with light coming down the fiber-optic, to insure that all of the light is falling inside the active area, within a telecommunication receiver. Difficulty involved in fiber-optic to APD alignment is increased when floating field rings are required, due to increased capacitance of a p-n junction.
There is also a continuous desire to increase performance characteristics of APDs. It is desirable that the APDs exhibit less noise, have increased gain, produce less surface leakage current, and have improved reliability.
It would therefore be desirable to develop an APD that provides low noise, low surface leakage current, high gain, uniform process yields, and is capable of responding to wavelengths within a desired range. It would also be desirable for the APD to exhibit less time consumption and have improved cost effectiveness in alignment with a fiber-optic cable within a telecommunication receiver.
The present invention provides a semiconductor avalanche photodiode for use in a transceiver and a method of fabricating the same. A recessed p-type region cap layer avalanche photodiode is provided. The photodiode includes a semiconductor substrate and a semiconductor stack, which is electrically coupled to the substrate. A cap layer is electrically coupled to the stack and includes a recessed p-type region. The recessed p-type region forms a p-n junction with the stack. A method of forming the photodiode is also provided. The method includes forming the substrate, the stack, and the cap layer. The cap layer is selectively etched to expose the stack and form a cap layer opening. Dopant is diffused through the cap layer opening into the stack to form the p-n junction.
One of several advantages of the present invention is that a single diffusion is performed into a controlled multiplication layer thickness. The single diffusion eliminates a need for floating field rings and/or guard rings and provides decreased p-n junction capacitance, thereby not limiting bandwidth of the photodiode.
Another advantage of the present invention is that it maintains process uniformity of epitaxial growth and diffusion of dopant over an entire substrate. A uniform diffusion front into the underlying InP multiplication layer is insured and results from a single preferential wet chemical etch that stops on the InP layer. Uniformity of epitaxial growth and diffusion of dopant results in relatively high process yields across the photodiode.
Furthermore, the cap layer of the present invention has a wide-bandgap and is passivated resulting in reduced surface leakage current and improved device reliability over avalanche photodiodes of prior art.
Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.